Uncertainty about predation risk: a conceptual review

Uncertainty has long been of interest to economists and psychologists and has more recently gained attention among ecologists. In the ecological world, animals must regularly make decisions related to finding resources and avoiding threats. Here, we describe uncertainty as a perceptual phenomenon of decision-makers, and we focus specifically on the functional ecology of such uncertainty regarding predation risk. Like all uncertainty, uncertainty about predation risk reflects informational limitations. When cues are available, they may be novel (i.e. unknown information), incomplete, unreliable, overly abundant and complex, or conflicting. We review recent studies that have used these informational limitations to induce uncertainty of predation risk. These studies have typically used either over-responses to novelty (i.e. neophobia) or memory attenuation as proxies for measuring uncertainty. Because changes in the environment, particularly unpredictable changes, drive informational limitations, we describe studies assessing unpredictable variance in spatio-temporal predation risk, intensity of predation risk, predator encounter rate, and predator diversity. We also highlight anthropogenic changes within habitats that are likely to have dramatic impacts on information availability and thus uncertainty in antipredator decisions in the modern world.

Olfaction and reaction: The role of olfactory and hypothalamic investment in the antipredator responses to chemical alarm cues by northern redbelly dace

Neuroplasticity enables teleosts to promote or downregulate the growth of their brains regionally. To compensate for the effects of predation pressure, teleosts may alter their brain morphology and behavioral responses to mitigate its impact on individual fitness. High-predation environments often promote specific patterns of brain growth and produce bolder and more proactive populations. Owing to the expense of maintaining neural tissue, relative size indicates the regions most relied upon. In northern redbelly dace Chrosomus eos, as little as 2 weeks of elevated predation pressure, resulted in increased investment in their olfactory bulbs and optic tecta, while the imposition of captivity produced smaller, less symmetric hypothalami. Taken together, these results suggest that an individual could potentially become better able to detect a threat, and simultaneously less inclined to react to it, making the impact of either change in isolation is difficult to discern. Here, we compared interindividual variation in gross brain morphology, risk-taking tactics in a novel arena (shy-bold personality), and responding to olfactory cues (proactive/reactive stress-coping style). We hypothesized that olfactory investment would positively correlate with response intensity to predator cue concentration and respond across a wider range of cue concentrations, while hypothalamus size would correlate with shyness and reactivity. Exposure to heightened risk produced more bold/proactive individuals, with larger olfactory bulbs and smaller hypothalami. However, the direction of the correlation between hypothalamus size and behavior varied by treatment, and olfactory investment only corresponded with response intensity amongst proactive individuals. Our findings illustrate the potential pitfalls of relating gross brain morphology to complex behavior and suggest that stress-coping style is a relevant consideration in future studies.

Microhabitat conditions drive uncertainty of risk and shape neophobic responses in Trinidadian guppies, Poecilia reticulata

In response to uncertain risks, prey may rely on neophobic phenotypes to reduce the costs associated with the lack of information regarding local conditions. Neophobia has been shown to be driven by information reliability, ambient risk and predator diversity, all of which shape uncertainty of risk. We similarly expect environmental conditions to shape uncertainty by interfering with information availability. In order to test how environmental variables might shape neophobic responses in Trinidadian guppies (Poecilia reticulata), we conducted an in situ field experiment of two high-predation risk guppy populations designed to determine how the 'average' and 'variance' of several environmental factors might influence the neophobic response to novel predator models and/or novel foraging patches. Our results suggest neophobia is shaped by water velocity, microhabitat complexity, pool width and depth, as well as substrate diversity and heterogeneity. Moreover, we found differential effects of the 'average' and 'variance' environmental variables on food- and predator-related neophobia. Our study highlights that assessment of neophobic drivers should consider predation risk, various microhabitat conditions and neophobia being tested. Neophobic phenotypes are expected to increase the probability of prey survival and reproductive success (i.e. fitness), and are therefore likely linked to population health and species survival. Understanding the drivers and consequences of uncertainty of risk is an increasingly pressing issue, as ecological uncertainty increases with the combined effects of climate change, anthropogenic disturbances and invasive species.

The role of donor and receiver size in the response to public cues in Hart’s rivulus, Anablepsoides hartii

Aquatic prey use chemical alarm cues as public information sources to optimize behavioural decisions. Recent studies suggest that the contextual value of these cues is shaped by their source, the size of the donor relative to the receiver, and the size of the receiver itself. Here, we exposed Hart’s rivulus (Anablepsoides hartii) to conspecific or heterospecific alarm cues from donors that were either smaller or larger than the mean focal rivulus size. Smaller rivulus reduced foraging in response to conspecific and heterospecific cues, regardless of donor size. However, larger rivulus exhibited no reduction in foraging towards small conspecific cues and increased foraging towards small heterospecific cues. Additionally, while conspecific donors elicited strong predator avoidance, rivulus exhibited stronger responses to large vs. small heterospecific cues. Our results demonstrate that the value of alarm cues is shaped by the interacting effects of receiver size and the size and species of cue donors.

Assessing effects of predator density and diversity on neophobia in Trinidadian guppies

Neophobic predator avoidance, where prey actively avoid novel stimuli, is thought to allow prey to cope with the inability to predict predation risk (i.e. uncertainty) while reducing the costs associated with learning. Recent studies suggest that neophobia is elicited as a response to unpredictable and elevated mean predation risk, and is linked to experience with diverse novel cues. However, no research has disentangled the effects of predator density and diversity on neophobia. We conditioned Trinidadian guppies (Poecilia reticulata) to high- or low-diversity predator model treatments paired with high, intermediate, or low concentrations of conspecific alarm cues as a proxy for predator density. We tested behavioural responses to a novel stimulus vs. a water control to determine differences in neophobia among treatments. We found that neophobic shoaling behaviour was shaped by mean risk (predator density). However both density and diversity shaped neophobic freezing, and to a weaker extent, neophobic area use. Our research suggests that predator diversity might elicit neophobic responses in guppies, but only when mean risk is high enough. The relationship between neophobia and components of predation risk is becoming increasingly relevant as ecological uncertainty becomes more prevalent with increasing climate change, anthropogenic impacts, and invasive species.

Mating competition and adult sex ratio in wild Trinidadian guppies

Most experimental tests of mating systems theory have been conducted in the laboratory, using operational sex ratios (ratio of ready-to-mate male to ready-to-mate female) that are often not representative of natural conditions. Here, we first measured the range of adult sex ratio (proportion of adult males to adult females; ASR) in two populations of Trinidadian guppies (Poecilia reticulata) differing in ambient predation risk (high vs. low). We then explored, under semi-wild conditions, the effect of ASR (i.e., 0.17, 0.50, 0.83) on mating competition patterns in these populations. ASR in the wild was female-biased and did not significantly differ between the two populations. The range of ASR in our experiment was representative of natural ASRs. As expected, we observed an increase in intrasexual aggression rates in both sexes as the relative abundance of competitors increased. In support of the risky competition hypothesis, all measured behaviors had lower rates in a high versus low predation-risk population, likely due to the costs of predation. In terms of mating tactics, a male-biased ASR did not lead males to favor forced mating over courtship, indicating that males did not compensate for the cost of competition by switching to a less costly alternative mating tactic. Overall, this study highlights the need for field experiments using natural ranges of ASRs to test the validity of mating systems theory in a more complex, ecologically relevant context.

Exploratory decisions of Trinidadian guppies when uncertain about predation risk

Animals can reduce their uncertainty of predation risk by gathering new information via exploration behaviour. However, a decision to explore may also be costly due to increased predator exposure. Here, we found contextual effects of predation risk on the exploratory activity of Trinidadian guppies Poecilia reticulata in a novel environment. First, guppies were exposed to a 3-day period of either high or low background predation risk in the form of repeated exposure to either injured conspecific cues (i.e. alarm cues) or control water, respectively. A day later, guppies were moved into a testing arena with limited visual information due to structural barriers and were then presented with an acute chemical stimulus, either alarm cues (a known and reliable indicator of risk), a novel odour (an ambiguous cue), or control water. In the presence of control water, guppies from high and low background risk showed a similar willingness to explore the arena. However, high-risk individuals significantly reduced their spatial evenness, although not their movement latency, in the presence of both the alarm and novel cues. When these high-risk individuals were a member of a shoal, they became willing to explore the environment more evenly in the presence of alarm cues while remaining cautious toward the novel cue, indicating an effect of the greater uncertainty associated with the novel cue. In contrast, low-risk guppies showed a willingness to explore the arena regardless of acute threat or social context. Such contextual effects of background risk and social context highlight the complexity of exploratory decisions when uncertain.

Predation risk shapes the use of conflicting personal risk and social safety information in guppies

When faced with uncertainty, animals can benefit from using multiple sources of information in order to make an optimal decision. However, information sources (e.g., social and personal cues) may conflict, while also varying in acquisition cost and reliability. Here, we assessed behavioral decisions of Trinidadian guppies (Poecilia reticulata), in situ, when presented with conflicting social and personal information about predation risk. We positioned foraging arenas within high- and low-predation streams, where guppies were exposed to a personal cue in the form of conspecific alarm cues (a known indicator of risk), a novel cue, or a control. At the same time, a conspecific shoal (a social safety cue) was either present or absent. When social safety was absent, guppies in both populations showed typical avoidance responses towards alarm cues, and high-predation guppies showed their typical avoidance of novel cues (i.e., neophobia). However, the presence of social safety cues was persuasive, overriding the neophobia of high-predation guppies and emboldening low-predation guppies to ignore alarm cues. Our experiment is one of the first to empirically assess the use of safety and risk cues in prey and suggests a threshold level of ambient risk which dictates the use of conflicting social and personal information.

Disturbance cue communication is shaped by emitter diet and receiver background risk in Trinidadian guppies

In animal communication systems, individuals that detect a cue (i.e., “receivers”) are often influenced by characteristics of the cue emitter. For instance, in many species, receivers avoid chemical cues that are released by emitters experiencing disturbance. These chemical “disturbance cues” appear to benefit receivers by warning them about nearby danger, such as a predator’s approach. While the active ingredients in disturbance cues have been largely unexplored, by-products of metabolized protein are thought to play a role for some species. If so, the content (quality) and volume (quantity) of the emitter’s diet should affect their disturbance cues, thus altering how receivers perceive the cues and respond. Guppies Poecilia reticulata are a species known to discriminate among disturbance cues from different types of donors, but dietary variation has yet to be explored. In this study, we found evidence that diet quality and quantity can affect disturbance cues released by guppy emitters (i.e., experimental “donors”). Receivers discriminated between donor cue treatments, responding more strongly to cues from donors fed a protein-rich bloodworm diet (Experiment 1), as well as an overall larger diet (Experiment 2). We also found that receivers exposed to higher background risk were more sensitive to disturbance cue variation, with the strongest avoidance responses displayed by high-risk receivers toward disturbance cues from donors fed the high-quality diet. Therefore, diet, and perhaps protein specifically, affects either the concentration or composition of disturbance cues released by guppies. Such variation may be important in information signaling in social species like the guppy.

Early-life and parental predation risk shape fear acquisition in adult minnows

Exposure to predation risk can induce a fearful baseline state, as well as fear reactions toward novel situations (i.e., neophobia). Some research indicates that risk exposure during sensitive periods makes adults more prone to acquiring long-term fearful phenotypes. However, chronic risk can also lead to ignoring threats in order to maintain other activities. We sought to assess how a relatively long period of low risk, experienced either early in life or by the previous generation, influences fear behaviour acquired from a short period of high risk as adults. We used fathead minnows as study subjects and simulated predation risk with repeated exposures to conspecific chemical alarm cues. The period of high risk experienced by adults induced typical fear behaviour (baseline freezing and neophobia), whereas the early-life low-risk period 1 year prior caused only a reduction in baseline foraging. We found no evidence that the early-life risk significantly altered the fear acquired from the adult-risk period. However, in a second experiment, a low-risk period during the parental generation interacted with a high-risk period experienced by the adult offspring. The combination of both risk periods heightened baseline freezing despite parental risk having little effect independently. Hence, our study provides evidence that parental risk exposure can lead to an additive intergenerational effect on fear acquisition in minnows.

Exploring the threat-sensitive predator avoidance hypothesis on mate competition in two wild populations of Trinidadian guppies

The intensity of mate competition is often influenced by predation pressure. The threat-sensitive predator avoidance hypothesis predicts that prey should precisely adjust their fitness-related activities to the level of perceived acute predation risk and this effect should be stronger under high background risk. Individuals should compensate during periods of moderate risk for lost opportunities during high risk. Our study examined the interaction between acute and background predation risk on mate competition. Under laboratory conditions, we explored the effects of acute risk (low vs. high) using chemical alarm cue (AC; control (results presented in Chuard et al. (2016)) The effects of adult sex ratio on mating competition in male and female guppies (Poecilia reticulata) in two wild populations. Behav Process 129:1-10), 25 % concentration, and 100 % concentration), and population of origin (low vs. high background risk) on mate competition in guppies (Poecilia reticulata). Surprisingly, males favored courtship over forced mating under acute predation risk irrespective of background risk, potentially benefiting from a female preference for bold males. We discuss our results in the context of chemical threat-sensitivity and resource differences in defendability (e.g. mates vs. food).

Short-term captivity drives hypothalamic plasticity and asymmetry in wild-caught northern red bellied dace (Chrosomus eos)

Teleost fish are neuroplastic and are known to alter their brain morphology and behaviour in response to environmental change such as an increase in predation pressure. The hypothalamus plays a key role in regulating behavioural responses to predation risk. In this study, wild-caught northern red bellied dace (Chrosomus eos) developed smaller and less symmetric hypothalami when held in captivity for 14 days; both measures correlated with boldness in a latency to emerge test. This study's results highlight the potential impact of short-term holding conditions on brains and behaviour.

Sender and receiver experience alters the response of fish to disturbance cues

Predation is a pervasive selection pressure, shaping morphological, physiological, and behavioral phenotypes of prey species. Recent studies have begun to examine how the effects of individual experience with predation risk shapes the use of publicly available risk assessment cues. Here, we investigated the effects of prior predation risk experience on disturbance cue production and use by Trinidadian guppies Poecilia reticulata under laboratory conditions. In our first experiment, we demonstrate that the response of guppies from a high predation population (Lopinot River) was dependent upon the source of disturbance cue senders (high vs. low predation populations). However, guppies collected from a low predation site (Upper Aripo River) exhibited similar responses to disturbance cues, regardless of the sender population. In our second experiment, we used laboratory strain guppies exposed to high versus low background risk conditions. Our results show an analogous response patterns as shown for our first experiment. Guppies exposed to high background risk conditions exhibited stronger responses to the disturbance cues collected from senders exposed to high (vs. low) risk conditions and guppies exposed to low risk conditions were not influenced by sender experience. Combined, our results suggest that experience with background predation risk significantly impacts both the production of and response to disturbance cues in guppies.

Rapid plastic changes in brain morphology in response to acute changes in predation pressure in juvenile Atlantic salmon (Salmo salar) and northern redbelly dace (Phoxinus eos)

Teleosts exhibit inter- and intra-specific variation in the size and shape of their brains. Interpopulation differences in gross brain morphology among numerous teleost fish species have been observed and have been partially attributed to plastic changes in response to their environment, including predation. These differences manifest themselves macroscopically, potentially because teleosts retain the capacity for active neuroproliferation into adulthood. Building on previous work, showing chronic exposure to predation can affect brain morphology, we sought to determine whether these differences manifest themselves on a time scale shown to induce phenotypically plastic behavioural changes. In separate trials, we held northern redbelly dace (Phoxinus eos (Cope, 1861) = Chrosomus eos Cope, 1861) and juvenile Atlantic salmon (Salmo salar Linnaeus, 1758) in semi-natural conditions and exposed them to conspecific skin extract as a proxy for predation risk over 2 weeks. After exposure, their brains were excised, photographed, and analyzed for size (multivariate ANOVA) and shape (Procrustes ANOVA). Despite their brief exposure to simulated predation pressure, subjects from both species developed significantly different brain morphologies. Compared with controls, the Atlantic salmon exhibited a different brain shape and smaller optic tecta, whereas the northern redbelly dace had larger brains with more developed olfactory bulbs and optic tecta. Our results highlight the rapidity with which external environment can alter patterns of growth in the brain.

An ecological framework of neophobia: from cells to organisms to populations

Neophobia is the fear of novel stimuli or situations. This phenotype has recently received much ecological attention, primarily in the context of decision making. Here, we explore neophobia across biological levels of organisation, first describing types of neophobia among animals and the underlying causes of neophobia, highlighting high levels of risk and uncertainty as key drivers. We place neophobia in the framework of Error Management Theory and Signal Detection Theory, showing how increases in overall risk and uncertainty can lead to costly non-responses towards novel threats unless individuals lower their response threshold and become neophobic. We then discuss how neophobic behaviour translates into population and evolutionary consequences before introducing neophobia-like processes at the cellular level, where some phenomena such as allergy and autoimmunity can parallel neophobic behaviour. Finally, we discuss neophobia attenuation, considering how a sudden change in the environment from dangerous to safe can lead to problematic over-responses (i.e. the 'maladaptive defensive carry-over' hypothesis), and discuss treatment methods for such over-responses. We anticipate that bridging the concept of neophobia with a process-centered perspective can facilitate a transfer of insight across organisational levels.

Microhabitat complexity influences fear acquisition in fathead minnows

Habitat varies in structure, with animals often preferring a certain degree of microhabitat complexity that facilitates fitness-related activities such as predator avoidance. Environments with high predation risk can induce elevated baseline fear and neophobia in prey, but whether microhabitat complexity influences the acquisition of neophobia has yet to be reported. Here, we tested whether exposure to predation risk induces different levels of fear in microhabitats that differed in complexity. We exposed fathead minnows, Pimephales promelas, to predation risk repeatedly (12 times over 4 days) in the form of damage-released chemical alarm cues (compared to water control) in tanks with vertical plant structure distributed either throughout the tank (complex habitat) or clumped together (simple habitat). Then, we tested minnows before and after exposure to a novel odor in tanks with either the same microhabitat complexity (i.e., familiar habitats) or in tanks with novel habitat that had different substrate structure and no vertical structure. Minnows in the complex habitat showed less overall movement one day after the background risk period, whereas individuals in the simple habitat showed reduced movement regardless of prior risk exposure. We observed stronger effects in the novel habitat, where background risk in both simple and complex habitats caused neophobia. However, individuals from the simple background habitat showed higher baseline fear behaviors. Hence, for minnows, low microhabitat complexity appears to lead to elevated fear, which remains even after a habitat change.

Does donor group size matter? The response of guppies (Poecilia reticulata) and convict cichlids (Amatitlania nigrofasciata) to disturbance cues from conspecific and heterospecific donors

Prey are under immense pressure to make context-specific, behavioural decisions. Prey use public information to reduce the costs associated with making inappropriate decisions. Chemical cues are commonly used by aquatic vertebrates to assess local threats and facilitate behavioural decision making. Previous studies on chemosensory assessment of risk have largely focused on damage-released alarm cues, with the cues released by disturbed or stressed prey (i.e., disturbance cues) receiving less attention. Disturbance cues are “early-warning signals” common among aquatic vertebrates that may warn conspecific and heterospecific prey guild members of potential risk. Initially, we conducted a series of laboratory studies to determine (i) if guppies (Poecilia reticulata Peters, 1859) produce and respond to disturbance cues and (ii) if relative concentration (donor group size) determines response intensity. Secondly, we examined if guppies and convict cichlids (Amatitlania nigrofasciata (Günther, 1867)) show similar response patterns to their own vs. heterospecific disturbance cues. Our results suggest that guppies exhibit increased predator avoidance behaviour to conspecific disturbance cues (relative to water from undisturbed conspecifics) and increased donor group size lead to stronger antipredator responses. However, although guppies and cichlids respond to each other’s disturbance cues, we found no effect of donor group size towards heterospecific disturbance cues. Our results suggest that disturbance cues are not generalized cues and present a degree of species-specificity.

Competition for food in 2 populations of a wild-caught fish

Aggressive behavior when competing for resources is expected to increase as the ratio of competitors-to-resource ratio (CRR) units increases. Females are expected to be more aggressive than males when competing for food when body size is more strongly related to reproductive success in females than in males, whereas aggression is predicted to decrease under high ambient predation risk by natural selection. Under the risk allocation model, however, individuals under high ambient predation risk are expected to be more aggressive, and forage more in the absence of imminent risk than their low risk counterparts. An interaction between adult sex ratio (i.e., adult males/females), ambient predation risk (high vs. low), and sex on intrasexual competition for mates in Trinidadian guppies Poecilia reticulata has been shown. The interaction suggested an increase in aggression rates as CRR increased, except for males from the high predation population. To compare the patterns of competition for food versus mates, we replicated this study by using food patches. We allowed 4 male or 4 female guppies from high and low predation populations to compete for 5, 3, or 1 food patches. The foraging rate was higher in a high rather than low ambient predation risk population. Surprisingly, CRR, sex, and population of origin had no effect on aggression rates. Despite other environmental differences between the 2 populations, the effect of ambient predation risk may be a likely explanation for differences in foraging rates. These results highlight the importance for individuals to secure food despite the cost of competition and predation.

Personality and the retention of neophobic predator avoidance in wild caught Trinidadian guppies

Neophobic predator avoidance allows prey to reduce the risk of predation but is costly in terms of reduced foraging or courtship opportunities if the novel cues do not represent an actual threat. Consequently, neophobic responses to novel cues should wane with repeated exposures in the absence of an actual threat. We tested the prediction that individual personality traits shape the retention of neophobic predator avoidance in wild-caught guppies. Using extinction trials, we demonstrate that personality (measured as latency to escape or approach a novel object) did not influence the initial response of wild-caught Trinidadian guppies to a novel odour; bolder and shyer guppies both exhibited similarly strong avoidance responses. However, after several exposures, shyer guppies maintain an avoidance response, and bolder guppies no longer respond. Our results highlight the complex nature of the antipredator algorithm of prey, whereby past experience, acute risk, and individual tactics shape neophobic predator avoidance patterns.

Migratory-stage sea lamprey Petromyzon marinus stop responding to conspecific damage-released alarm cues after 4 h of continuous exposure in laboratory conditions

This study investigated the length of avoidance response of migratory-stage sea lamprey Petromyzon marinus exposed continuously to conspecific damage-released alarm cues for varying lengths of time in laboratory stream channels. Ten replicate groups of P. marinus, separated by sex, were exposed to either deionized water control or to P. marinus extract for 0, 2 or 4 h continuously. Petromyzon marinus maintained their avoidance response to the conspecific damage-released alarm cue after continuous exposure to the alarm cue for 0 and 2 h but not 4 h. Beyond being one of the first studies in regards to sensory-olfactory adaptation-acclimation of fishes to alarm cues of any kind, these results have important implications for use of conspecific alarm cues in P. marinus control. For example, continuous application of conspecific alarm cue during the day, when P. marinus are inactive and hiding, may result in sensory adaptation to the odour by nightfall when they migrate upstream.

Local predation risk shapes spatial and foraging neophobia patterns in Trinidadian guppies

The "dangerous niche" hypothesis posits that neophobia functions to reduce the cost of habitat use among animals exposed to unknown risks. For example, more dangerous foraging or higher competition may lead to increased spatial neophobia. Likewise, elevated ambient predation threats have been shown to induce phenotypically plastic neophobic predator avoidance. In both cases, neophobia is argued to reduce the cost of living associated with ecological uncertainty. Here, we test the hypothesis that ambient predation shapes both neophobic predator avoidance and spatial and foraging neophobia in Trinidadian guppies. Guppies were exposed to a novel foraging arena paired with a known cue (conspecific alarm cue), a novel cue (lemon odor), or a stream water control in three streams differing in ambient predation risk. We demonstrate that guppies from a high-predation-risk stream exhibited risk-averse foraging patterns regardless of the chemical stimulus presented (high spatial neophobia) and that those from a low-predation-risk stream were only risk-averse when the foraging arenas were paired with conspecific alarm cue (lower spatial neophobia). Those tested in the intermediate-predation-risk stream were consistently intermediate to the high-risk vs. low-risk populations. Our study suggests that ambient predation risk shapes both neophobic predator avoidance and space-use patterns and that neophobia may function as a "generalized" response to ecological uncertainty.

Risk-induced neophobia: does sensory modality matter?

Recent studies have documented that exposure to high levels of background risk can induce neophobic predator avoidance in prey animals, whereby they respond to any novel cue with an anti-predator response. Such phenotypically plastic predator avoidance may allow prey to maximize anti-predator benefits in variable risk environments. It remains poorly understood whether risk assessment information from different sensory modalities can be integrated to induce generalized, cross-sensory system neophobic responses. Here, we directly test this hypothesis by exposing juvenile convict cichlids (Amatitlania nigrofasciata) to high- versus low-risk environments using either conspecific alarm cue (chemosensory risk) or a model avian predator (visual/mechanical risk) and testing their response to a novel chemosensory cue (Experiment 1) or visual cue (Experiment 2). Our results suggest that regardless of the sensory modality used to increased perceived risk, cichlids pre-exposed to high-risk conditions exhibited increased predator avoidance in response to any novel visual or chemical cue. As expected, cichlids pre-exposed to low-risk conditions did not display any neophobic responses. Our results suggest that induced neophobia is not cue specific; rather, it may function as a generalized response to perceived predation risk.

Interactive effects of reproductive assets and ambient predation risk on the threat-sensitive decisions of Trinidadian guppies

Threat-sensitive behavioral trade-offs allow prey animals to balance the conflicting demands of successful predator detection and avoidance and a suite of fitness-related activities such as foraging, mating, and territorial defense. Here, we test the hypothesis that background predation level and reproductive status interact to determine the form and intensity of threat-sensitive behavioral decisions of wild-caught female Trinidadian guppies Poecilia reticulata. Gravid and nongravid guppies collected from high- and low-predation pressure populations were exposed to serial dilutions of conspecific chemical alarm cues. Our results demonstrate that there was `no effect of reproductive status on the response of females originating from a low-predation population, with both gravid and nongravid guppies exhibiting strong anti-predator responses to the lowest concentration of alarm cues tested. Increasing cue concentrations did not result in increases in response intensity. Conversely, we found a significant effect of reproductive status among guppies from a high-predation population. Nongravid females from the high-predation population exhibited a strong graded (proportional) response to increasing concentrations of alarm cue. Gravid females from the same high-predation population, however, shifted to a nongraded response. Together, these results demonstrate that accrued reproductive assets influence the threat-sensitive behavioral decisions of prey, but only under conditions of high-ambient predation risk.

Evolution of the macromolecular structure of sporopollenin during thermal degradation

Reconstructing the original biogeochemistry of organic microfossils requires quantifying the extent of the chemical transformations they experienced during burial and maturation processes. In the present study, fossilization experiments have been performed using modern sporopollenin chosen as an analogue for the resistant biocompounds possibly constituting the wall of many organic microfossils. Sporopollenin powder has been processed thermally under argon atmosphere at different temperatures (up to 1000 °C) for varying durations (up to 900 min). Solid residues of each experiment have been characterized using infrared, Raman and synchrotron-based XANES spectroscopies. Results indicate that significant defunctionalisation and aromatization affect the molecular structure of sporopollenin with increasing temperature. Two distinct stages of evolution with temperature are observed: in a first stage, sporopollenin experiences dehydrogenation and deoxygenation simultaneously (below 500 °C); in a second stage (above 500 °C) an increasing concentration in aromatic groups and a lateral growth of aromatic layers are observed. With increasing heating duration (up to 900 min) at a constant temperature (360 °C), oxygen is progressively lost and conjugated carbon–carbon chains or domains grow progressively, following a log-linear kinetic behavior. Based on the comparison with natural spores fossilized within metasediments which experienced intense metamorphism, we show that the present experimental simulations may not perfectly mimic natural diagenesis and metamorphism. Yet, performing such laboratory experiments provides key insights on the processes transforming biogenic molecules into molecular fossils.

Retention of neophobic predator recognition in juvenile convict cichlids: effects of background risk and recent experience

Exposure to conditions of elevated predation risk, even for relatively short periods, has been shown to induce neophobic responses to novel predators. Such phenotypically plastic responses should allow prey to exhibit costly anti-predator behaviour to novel cues only in situations where the risk of predation is high. While there is evidence that the level of background risk shapes the strength of induced neophobia, we know little about how long neophobic responses are retained. Here we exposed juvenile convict cichlids (Amatitlania nigrofasciata) to three background levels of short-term background risk and then tested their responses to novel predator odours. Cichlids exposed to low risk did not show neophobic responses, while those exposed to intermediate and high risk did. Using extinction trials, we demonstrate that the retention of neophobic responses is greater among cichlids exposed to high versus intermediate predation risk conditions. Moreover, we found much longer retention of the neophobic responses when cichlids were tested a single time compared to when they were tested repeatedly in the extinction trials. This work supports the prediction that neophobic responses to specific odours are relatively long lasting but can quickly wane if the cues are experienced repeatedly without them being associated with risk. It is clear that background level of risk and the frequency of exposure to novel cues are crucial factors in determining the retention of risk-related information among prey.

Getting ready for invasions: can background level of risk predict the ability of naïve prey to survive novel predators?

Factors predicting the outcome of predator invasions on native prey communities are critical to our understanding of invasion ecology. Here, we tested whether background level of risk affected the survival of prey to novel predators, both native and invasive, predicting that high-risk environments would better prepare prey for invasions. We used naïve woodfrog as our prey and exposed them to a high or low risk regime either as embryos (prenatal exposure) or as larvae (recent exposure). Tadpoles were then tested for their survival in the presence of 4 novel predators: two dytiscid beetles, crayfish and trout. Survival was affected by both risk level and predator type. High risk was beneficial to prey exposed to the dytiscids larvae (ambush predators), but detrimental to prey exposed to crayfish or trout (pursuit predators). No effect of ontogeny of risk was found. We further documented that high-risk tadpoles were overall more active than their low-risk counterparts, explaining the patterns found with survival. Our results provide insights into the relationship between risk and resilience to predator invasions.

Time-sensitive neophobic responses to risk

Prey animals that experience a high background level of risk are known to exhibit considerable phenotypic plasticity in their responses to unknown predators. When background risk is high, prey exhibit neophobic responses to unknown odours, i.e. they show a fear response to any new stimulus. Here, we examine whether temporal variation in the pattern of risk to which prey are exposed influences neophobic responses. To establish prey groups with different temporal patterns of risk, embryonic woodfrogs (Lithobates sylvaticus) were exposed to conspecific alarm cues each morning and control cues in the evening, or conspecific alarm cues each evening and control cues in the morning, for their entire embryonic period. After the tadpoles hatched they were tested at both times of day for known risk cues (alarm cues), unknown predator odours or water control. Tadpoles responded to alarm cues at any time of day, but showed neophobic responses to predator odours only if their test time matched their embryonic risk exposure time. These results demonstrate a high level of sophistication of neophobic responses and points to temporal variation in risk as a key driver of antipredator decision making.

Compensatory foraging in Trinidadian guppies: Effects of acute and chronic predation threats

In response to acute predation threats, prey may sacrifice foraging opportunities in favour of increased predator avoidance. Under conditions of high or frequent predation risk, such trade-offs may lead to reduced fitness. Here, we test the prediction that prey reduce the costs associated with lost opportunities following acute predation threats by exhibiting short-term compensatory foraging responses. Under semi-natural conditions, we exposed female guppies Poecilia reticulate from high and low predation risk sites to one of three levels of acute predation threat (high, intermediate or low concentrations of conspecific alarm cues). Our results confirm previous reports, demonstrating that guppies from a high predation site were consistently ‘bolder’ (shorter escape latencies) and exhibited graded threat-sensitive responses to different simulated threat levels while those from the low predation site were ‘shyer’ and exhibited non-graded responses. Most importantly, we found that when guppies from low predation sites resumed foraging, they did so at rates significantly lower than baseline rates. However, guppies from high predation sites resumed foraging either at rates equal to baseline (in response to low or intermediate risk stimuli) or significantly increased relative to baseline rates (in response to high risk stimuli). Together, these results highlight a complex compensatory behavioral mechanism that may allow prey to reduce the long-term costs associated with predator avoidance.

The behavioural response of adult Petromyzon marinus to damage-released alarm and predator cues

Using semi-natural enclosures, this study investigated (1) whether adult sea lamprey Petromyzon marinus show avoidance of damage-released conspecific cues, damage-released heterospecific cues and predator cues and (2) whether this is a general response to injured heterospecific fishes or a specific response to injured P. marinus. Ten replicate groups of 10 adult P. marinus, separated by sex, were exposed to one of the following nine stimuli: deionized water (control), extracts prepared from adult P. marinus, decayed adult P. marinus (conspecific stimuli), sympatric white sucker Catostomus commersonii, Amazon sailfin catfish Pterygoplichthys pardalis (heterospecific stimuli), 2-phenylethylamine (PEA HCl) solution, northern water snake Nerodia sipedon washing, human saliva (predator cues) and an adult P. marinus extract and human saliva combination (a damage-released conspecific cue and a predator cue). Adult P. marinus showed a significant avoidance response to the adult P. marinus extract as well as to C. commersonii, human saliva, PEA and the adult P. marinus extract and human saliva combination. For mobile P. marinus, the N. sipedon washing induced behaviour consistent with predator inspection. Exposure to the P. pardalis extract did not induce a significant avoidance response during the stimulus release period. Mobile adult female P. marinus showed a stronger avoidance behaviour than mobile adult male P. marinus in response to the adult P. marinus extract and the adult P. marinus extract and human saliva combination. The findings support the continued investigation of natural damage-released alarm cue and predator-based repellents for the behavioural manipulation of P. marinus populations in the Laurentian Great Lakes.

Microbially enhanced dissolution of HgS in an acid mine drainage system in the California Coast Range

Mercury sulfides (cinnabar and metacinnabar) are the main ores of Hg and are relatively stable under oxic conditions (Ksp = 10⁻⁵⁴ and 10⁻⁵² , respectively). However, until now their stability in the presence of micro-organisms inhabiting acid mine drainage (AMD) systems was unknown. We tested the effects of the AMD microbial community from the inoperative Hg mine at New Idria, CA, present in sediments of an AMD settling pond adjacent to the main waste pile and in a microbial biofilm on the surface of this pond, on the solubility of crystalline HgS. A 16S rRNA gene clone library revealed that the AMD microbial community was dominated by Fe-oxidizing (orders Ferritrophicales and Gallionellas) and S-oxidizing bacteria (Thiomonas sp.), with smaller amounts (≤ 6%) being comprised of the orders Xanthomondales and Rhodospirillales. Though the order Ferritrophicales dominate the 16S rRNA clones (>60%), qPCR results of the microbial community indicate that the Thiomonas sp. represents ~55% of the total micro-organisms in the top 1 cm of the AMD microbial community. Although supersaturated with respect to cinnabar and metacinnabar, microcosms inoculated with the AMD microbial community were capable of releasing significantly more Hg into solution compared to inactivated or abiotic controls. Four different Hg-containing materials were tested for bacterially enhanced HgS dissolution: pure cinnabar, pure metacinnabar, mine tailings, and calcine material (processed ore). In the microcosm with metacinnabar, the presence of the AMD microbial community resulted in an increase of dissolved Hg concentrations up to 500 μg L ⁻¹during the first 30 days of incubation. In abiotic control microcosms, dissolved Hg concentrations did not increase above 100 ng L⁻¹ . When Hg concentrations were below 50 μg L⁻¹ , the Fe-oxidizing bacteria in the AMD microbial community were still capable of oxidizing Fe(II) to Fe(III) in the AMD solution, whereas concentrations above 50 μg L⁻¹ resulted in inhibition of microbial iron oxidation. Our experiments show that the AMD microbial community contributes to the dissolution of mercury sulfide minerals. These findings have major implications for risk assessment and future management of inoperative Hg mines worldwide.

Effects of acidification on olfactory-mediated behaviour in freshwater and marine ecosystems: a synthesis

For many aquatic organisms, olfactory-mediated behaviour is essential to the maintenance of numerous fitness-enhancing activities, including foraging, reproduction and predator avoidance. Studies in both freshwater and marine ecosystems have demonstrated significant impacts of anthropogenic acidification on olfactory abilities of fish and macroinvertebrates, leading to impaired behavioural responses, with potentially far-reaching consequences to population dynamics and community structure. Whereas the ecological impacts of impaired olfactory-mediated behaviour may be similar between freshwater and marine ecosystems, the underlying mechanisms are quite distinct. In acidified freshwater, molecular change to chemical cues along with reduced olfaction sensitivity appear to be the primary causes of olfactory-mediated behavioural impairment. By contrast, experiments simulating future ocean acidification suggest that interference of high CO2 with brain neurotransmitter function is the primary cause for olfactory-mediated behavioural impairment in fish. Different physico-chemical characteristics between marine and freshwater systems are probably responsible for these distinct mechanisms of impairment, which, under globally rising CO2 levels, may lead to strikingly different consequences to olfaction. While fluctuations in pH may occur in both freshwater and marine ecosystems, marine habitat will remain alkaline despite future ocean acidification caused by globally rising CO2 levels. In this synthesis, we argue that ecosystem-specific mechanisms affecting olfaction need to be considered for effective management and conservation practices.

Phenotypically plastic neophobia: a response to variable predation risk

Prey species possess a variety of morphological, life history and behavioural adaptations to evade predators. While specific evolutionary conditions have led to the expression of permanent, non-plastic anti-predator traits, the vast majority of prey species rely on experience to express adaptive anti-predator defences. While ecologists have identified highly sophisticated means through which naive prey can deal with predation threats, the potential for death upon the first encounter with a predator is still a remarkably important unresolved issue. Here, we used both laboratory and field studies to provide the first evidence for risk-induced neophobia in two taxa (fish and amphibians), and argue that phenotypically plastic neophobia acts as an adaptive anti-predator strategy for vulnerable prey dealing with spatial and temporal variation in predation risk. Our study also illustrates how risk-free maintenance conditions used in laboratory studies may blind researchers to adaptive anti-predator strategies that are only expressed in high-risk conditions.

The effect of turbidity on recognition and generalization of predators and non-predators in aquatic ecosystems

Recent anthropogenic activities have caused a considerable change in the turbidity of freshwater and marine ecosystems. Concomitant with such perturbations are changes in community composition. Understanding the mechanisms through which species interactions are influenced by anthropogenic change has come to the forefront of many ecological disciplines. Here, we examine how a change in the availability of visual information influences the behavior of prey fish exposed to potential predators and non-predators. When fathead minnows, Pimephales promelas, were conditioned to recognize predators and non-predators in clear water, they showed a highly sophisticated ability to distinguish predators from non-predators. However, when learning occurred under conditions of increased turbidity, the ability of the prey to learn and generalize recognition of predators and non-predators was severely impaired. Our work highlights that changes at the community level associated with anthropogenic perturbations may be mediated through altered trophic interactions, and highlights the need to closely examine behavioral interactions to understand how species interactions change.

Temperature-mediated changes in rates of predator forgetting in woodfrog tadpoles

Hundreds of studies have investigated the sources and nature of information that prey gather about their predators and the ways in which prey use this information to mediate their risk of predation. However, relatively little theoretical or empirical work has considered the question of how long information should be maintained and used by prey animals in making behavioural decisions. Here, we tested whether the size of the memory window associated with predator recognition could be affected by an intrinsic factor, such as size and growth rate of the prey. We maintained groups of predator-naive woodfrog, Lithobates sylvaticus, tadpoles at different temperatures for 8 days to induce differences in tadpole size. We then conditioned small and large tadpoles to recognize the odour of a predatory tiger salamander, Ambystoma tigrinum. Tadpoles were then maintained either on a high or low growth trajectory for another 8 days, after which they were tested for their response to the predator. Our results suggest that the memory window related to predator recognition of tadpoles is determined by both their size and/or growth rate at the time of learning and their subsequent growth rate post-learning.

Understanding the importance of episodic acidification on fish predator-prey interactions: does weak acidification impair predator recognition?

The ability of prey to recognize predators is a fundamental prerequisite to avoid being eaten. Indeed, many prey animals learn to distinguish species that pose a threat from those that do not. Once the prey has learned the identity of one predator, it may generalize this recognition to similar predators with which the prey has no experience. The ability to generalize reduces the costs associated with learning and further enhances the ability of the prey to avoid relevant threats. For many aquatic organisms, recognition of predators is based on odor signatures, consequently any anthropogenic alteration in water chemistry has the potential to impair recognition and learning of predators. Here we explored whether episodic acidification could influence the ability of juvenile rainbow trout to learn to recognize an unknown predator and then generalize this recognition to a closely related predator. Trout were conditioned to recognize the odor of pumpkinseed sunfish under circumneutral (~pH 7) conditions, and then tested for recognition of pumpkinseed or longear sunfish under both neutral or weakly acidic (~pH 6) conditions. When tested for a response to pumpkinseed odor, we found no significant effect of predator odor pH: trout responded similarly regardless of pH. Moreover, under neutral conditions, trout were able to generalize their recognition to the odor of longear sunfish. However, the trout could not generalize their recognition of the longear sunfish under acidic conditions. Given the widespread occurrence of anthropogenic acidification, acid-mediated impairment of predator recognition and generalization may be a pervasive problem for freshwater salmonid populations and other aquatic organisms.

Sensory complementation and antipredator behavioural compensation in acid-impacted juvenile Atlantic salmon

Prey incorporate multiple forms of publicly available information on predation risk into threat-sensitive antipredator behaviours. Changes in information availability have previously been demonstrated to elicit transient alterations in behavioural patterns, while the effects of long-term deprivation of particular forms of information remain largely unexplored. Damage-released chemical alarm cues from the epidermis of fishes are rendered non-functional under weakly acidic conditions (pH < 6.6), depriving fish of an important source of information on predation risk in acidified waterbodies. We addressed the effects of long-term deprivation on the antipredator responses to different combinations of chemical and visual threat cues via in situ observations of wild, free-swimming 0(+) Atlantic salmon (Salmo salar) fry in four neutral and four weakly acidic nursery streams. In addition, a cross-population transplant experiment and natural interannual variation in acidity enabled the examination of provenance and environment as causes of the observed differences in response. Fish living under weakly acidic conditions demonstrate significantly greater or hypersensitive antipredator responses to visual cues compared to fish under neutral conditions. Under neutral conditions, fish demonstrate complementary (additive or synergistic) effects of paired visual and chemical cues consistent with threat-sensitive responses. Cross-population transplants and interannual comparisons of responses strongly support the conclusion that differences in antipredator responses between neutral and weakly acidic streams result from the loss of chemical information on predation risk, as opposed to population-derived differences in behaviours.